Why Velux Windows Leak: Flashing Kits and Fitting

Start with the pattern. A leak follows the weather: it appears or gets worse during and after rain, usually at one edge of the frame, and dries out when the weather does. Condensation follows the temperature and the household. It shows on a cold morning, covers the whole pane or the lower frame, and turns up on dry days as well.

What does condensation look like on a roof window?

Beads of water across the glass, a wet bottom rail, and sometimes dark mould on the lining. It is the warm, damp air of the room meeting a cold surface. Cooking, showers and drying washing indoors add to it. Opening the window for a few minutes and keeping the room warm both help.

What if the glass is misted inside the panes?

That is a different problem. Mist trapped between two sheets of glass means the seal of the sealed unit has failed, and it will not wipe off. It is not a roof leak, and it does not mean water is entering the house. We explain it in why a Velux window mists between the panes.

A third look-alike sits in the loft rather than the room. In a cold roof under HR underlay, which does not let moisture through, Marley's ventilation guidance asks for 25mm of ventilation gap at the eaves where the pitch is 15° or less, and 10mm where it is steeper. Where that airflow is missing, damp gathers on cold surfaces near the window and drips back. Here the cause is the air movement, and the frame is innocent.

Flashing is the metal and fabric that bridges the gap between the window frame and the roof covering. On a roof window it comes as a kit of pieces for the bottom, sides and head, and each kit suits one type of covering. A kit built to sit under plain tiles is not the same as one built to sit on a deep profile.

Does the covering change the kit?

Yes. Slates and flat plain tiles lie thin and flat, so the flashing can be thin and flat. Interlocking concrete tiles are deeper, and so is the Double Roman, a broad interlocking profile with a rounded roll, so the flashing must follow their shape. Clay pantiles have a wave that needs a profile-matched apron at the base and sides. Mix the wrong kit with the wrong covering and the gaps that result are the ones water finds.

Our guide to Velux flashing kits for slate, tile and clay roofs sets out what to ask about for each covering. For the clay tiles made around Bridgwater, see Double Roman and pantile roofs in Somerset.

What about lead?

Some jobs, especially on older slate or in conservation settings, call for lead flashings dressed by hand around the frame. Lead is sold in numbered codes that reflect its thickness, and MyJobQuote notes that lead can last for over 80 years. Leadwork is slower than a factory kit and usually costs more, but it can suit a historic roof better. Our lead work page explains the trade.

Look at the roof from the side and you see three layers. Tiles or slates on the outside. Battens, the timber strips they hang on, below. Underlay, a membrane stretched over the rafters beneath those battens, which catches whatever the covering lets by. A roof window has to be integrated with all three, not just the top one.

What is an underlay collar?

It is a piece of fabric or membrane that wraps the frame and carries any water that gets past the tiles down and out over the covering below, instead of letting it run behind the window. If it is missing, torn, folded back or stuck to the wrong surface, rain that slips under the tiles can only find its way inside. It is easy to miss behind a neat-looking flashing.

Why does the underlay matter if the flashing is fine?

Because wind-driven rain goes under tiles and slates all the time, even on a sound roof. That is what the underlay is for. If the sheet around the frame has been cut short to make the window fit, water passes the flashing and lands on the frame's side. A repair that only replaces the flashing can leave that hidden fault in place.

Because HR underlay does not breathe, and old bitumen felt behaves the same way, a cold loft under either relies on air coming in at the eaves. Breathable membranes behave differently, and the type in place affects how the collar is lapped. We identify what is on your roof before deciding what the window needs.

Windows that have been in a Somerset roof for a long time tend to fail in a handful of ways. Knowing them helps you describe the problem when you ring.

What fails first on an old roof window?

The seals harden and gap, so wind-driven rain gets past them at the corners. Timber frames take water at the bottom rail and soften, and paint hides the damage until a screwdriver goes in. Metal aprons and side pieces dent or lift. The sealed glass unit mists. Old lead or mortar patches around the frame crack. Any of these is a reason for a closer look; none of them tells you by itself whether the whole window has to go.

When is repair enough?

When the frame is sound, the glass is clear and the fault is in the flashing or a few tiles at the side, a repair can be the better buy. When the timber is soft, the unit has misted and the flashing is of the wrong type, a new window and kit are more sensible than a patch. We tell you which applies, using photographs taken on the day so you can judge for yourself.

Our roof window and skylight installation page describes how a replacement is carried out. MyJobQuote lists around £1,100 as a typical UK figure for a standard mid-range Velux, with a straightforward swap around £100 less (MyJobQuote, Sep 2026).

Good installation shows in small decisions that nobody sees once the job is finished.

Where in the slope does the window sit?

Water running down the roof has to get around the window, so the area above it matters. A window placed directly below a valley, a chimney or a long unbroken run of tiles collects more water at its head than one sitting in a clear stretch. Where space is short, the head flashing needs extra care, and sometimes a small channel above the window is added to divert flow around it.

Does the frame have to be tight to the battens?

It must be fixed to the rafters at the right height and lined up with the covering so tiles lap over the side pieces without lifting. A frame set too high or too low upsets every lap. The battens around the frame need to be sound too, because they hold the kit down. Some of the tidy work is inside, where the lining and the insulation around the frame must be closed up so warm air does not carry moisture into the gap.

Why do the surrounding tiles matter?

The courses either side must be cut and fixed so wind cannot lift them, and any cracked or slipped neighbour should be replaced at the same time. Our post on finding and fixing leaks in a tiled pitched roof explains how water travels between the covering and the ceiling, so the stain seldom sits directly beneath the fault.

If water is coming in, protect the room first. Put a bucket or tray under the drip and move furniture, electrical items and rugs. If the ceiling is bulging, keep clear of it and call someone. Where water has reached a light fitting or socket, isolate that circuit at the consumer unit. Staying off a wet roof yourself is the safest choice.

What can you photograph from inside?

The frame, the lining and the ceiling, with a date on your phone, and the loft side of the window if you can reach it safely from a boarded area. Jot down which way the wind was blowing and how heavy the rain was when the water appeared. Those notes tell us where to look first.

Who do you tell first?

Tell your insurer, then set up a temporary repair that limits further damage, and keep photos and receipts. Payment turns on the terms of the policy and on what caused the leak, so read your cover and put the question to the insurer. Our answer on insurance and a leaking Velux goes through the order.

On a call-out, stopping the water comes ahead of everything else. Then we examine the flashing kit, the underlay round the frame and the covering, and you see the findings in photographs taken at your property. Should the window need replacing instead of repairing, the reason is clear to you before any price appears.